When an external force is given to a mass substance, acceleration is caused with the mass substance, at the same time an inertial force is observed for trying to retain the mass substance in the same location. The force given to a mass substance and an acceleration can be measured through a physical strain created by the inertial force. There have been a number of teachings on the method of detecting the strain. For example, a publication (Acceleration Sensor by Semiconductor, by Toshitaka SHIBATA, "Technology for Sensor", Vol. 11, No. 10, October, 1991) discloses a sensor formed on a silicon material, which sensor being manufactured using semiconductor technologies and micro-machining technologies. Means for detecting the strain includes the one which makes use of the change in the electrostatic capacitance, and the one which makes use of the piezoresistance effect, for example. Acceleration sensors using a single crystal silicon for the sensing portion have a stable performance and are capable of measuring the acceleration at high precision level. Therefore, the sensors of the above category form the mainstream in the recent field of acceleration sensors.
Processing a silicon substrate with a high precision level into a certain specific shape, however, needs the help of the high-level semiconductor technologies and the micro-machining technologies. Therefore, acceleration sensors are costly at present. Conventional sensors, as taught in a publication (Micromechanical Acceleromater Integrated with MOS Detection Circuitry, KURTE, PETERSEN etc. IEEE TRANSACTION ON ELECTRON DEVICES, VOL. ED-29, No 1, JANUARY 1982) and other publications, contain an etching-stop layer in the silicon substrate, and the monocrystalline silicon has to be epitaxially grown. These are the essential requirements to be fulfilled in order to form a hinge portion, or the portion strained by an acceleration, of a sensor. Expensive semiconductor manufacturing facilities are needed to carry out the above described process steps.
Japanese Laid-open Patent No.05-340957 discloses a method of joining an auxiliary glass substrate to a silicon substrate forming the hinge portion by anode bonding. However, the anode bonding method requires a facility for providing electric fields among the bonding substrates. Furthermore, the anode bonding is available only with a glass that contains sodium. The anode bonding method is not simple to practice, and restrictive in selecting the glass material.
Another type of acceleration sensor uses a piezoelectric element for the portion at which a strain is caused by acceleration. However, it is difficult for an acceleration sensor of the above type to measure a static acceleration; such as the measurement of an inclination relative to the gravity of the earth. In some cases, an acceleration sensor is required to have a self-diagnostic function. Most of such self-diagnostic functions require to have complicated peripheral circuits.
The present invention addresses the above described drawbacks, and aims to offer an inexpensive, yet highly accurate, acceleration sensor formed of silicon and other materials, without asking for the help of the high level semiconductor technologies or the micro-machining technologies. The acceleration sensor in accordance with the present invention covers a wide range of the frequency in acceleration measurement, and is capable of making a self-diagnosis about a disorder. The present invention also offers a method for manufacturing such acceleration sensors.